Apparently, the time you could cross the Rio Grande by boat for a couple of
bucks and either walk or ride on a burro or mule up to the town of Boquillas
del Carmen for a cerveza or two has passed - perhaps permanently.
Chalk it up to the current fear gripping the country, shamefully
exploited by our politicians, each one claiming to be tougher on terrorism than
the next. No, siree. We ain't gonna allow old Osama Ben to
enter the U.S. at Boquillas Crossing posing as a thirsty geologist. And
don't even think about buying a gorgeous
fluorite crystal or gaily
decorated hiking stick snuck across the river by the citizens of Boquillas, who
are trying to survive after the
collapse of the tourist trade. That there's
contraband, you know. (The Boquillastas will tell you the fluorite is
amethyst, but at two
dollars a crystal, it's still a bargain. Not that I
would advise you to break the law and buy it, you understand.)

Update: There is a project afoot to enable tourists to once
again visit and shop in Boquillas del Carmen and to allow the unfortunate
populace of the town to be able to come across the river to buy needed staples,
rather than having to go a hundred miles to the nearest legal crossing
(Presidio, TX). I understand the population of the town has plummeted, due to
lack of income and the needlessly imposed isolation. Even so, to placate those
who are still afraid the ghost of Osama might cross there, the border guys will
install some sort of security check. The vicious comments appended to
the online news stories by right wingers who have never been to the area were
ridiculous. According to them we can expect a flood of illegals crossing at
Boquillas. Well, there better be a bus service for them, because if they have
to cross Big Bend by foot, we will have a flood of dead illegals lying
across the park.

New Update, April, 2015: Even the anti-immigrant super
patriots should be pleased with the new crossing. (Old Mad Magazine
definition of a super patriot: "A person who loves his country while hating
90% of those who live in it.") You have to go through scrutiny by officials on
both sides to cross, and you'll need your US passport to get into Mexico. This
is a lot different from the old informal way crossings were allowed and no
doubt still a burden on the citizens of Boquillas del Carmen. As a ranger
pointed out, these citizens are our best defense against those crossing who
could be threats. They would quickly recognize a suspicious person, and they
would report it. It is in their interest, very much so, not to allow an
incident to lead to a closing of the border there.

The road to Boquillas Canyon is on the east side of the Chisos Mountains and
roughly follows Tornillo Creek. Tornillo ("screw" in Spanish) Creek gets its
name from a species of mesquite with seeds contained in a screw-shaped pod.
It's also known as "screw bean".

The image below was taken just after crossing a long bridge over troubled
(at infrequent times) water. Tornillo Creek can become a raging torrent. So,
although the bridge may appear to be a monument to pork-barrel spending (like
the Alaskan "bridge to nowhere"), it is actually a necessity if the road is not
to be washed away from time to time.

What you see in the above picture is an
igneousdike that penetrated
the Upper Cretaceous
(about 85 million year old) Pen
Formation, consisting largely of limey clays deposited in a shallow sea. The
Pen Formation lies on top of limestones deposited in deeper waters and is
overlain, where not exposed by erosion, by the Aguja formation, which grades
stratigraphically upward from marine to terrestrial deposits into the
terrestrial Javelina Formation. Hence the Pen Formation marks the beginning of
a transition in Big Bend history, when the sea was gradually receding as the
land rose, eventually to be visited by volcanism during the Eocene, beginning
around 50 million years ago. The dike, however, may have been emplaced during a
later volcanic episode, some 30 million years ago. The discoloration of the Pen
around the dike is due to the rocks being baked by the hot magma.

We saw a couple of fine dust devils during our Big Bend trip. Unfortunately,
I don't have a picture of the el primer one, because my brother kept
trying to maneuver his car to get the best perspective. Dust devils don't last
long, as I pointed out to no avail, so we ended up with a great perspective of
an empty desert scene. (Grrrr!) The one below wasn't bad but not nearly as
impressive as the huge one we had seen the first day we were there. I should
point out that the dust devil is a lot farther away - and therefore much larger
- than it appears in the image.

Hiking up the trail to the entrance to Boquillas canyon, you are treated to
some nice views of the Rio Grande with scenes of
fluvial geomorphology
(landforms associated with rivers and streams). In the view below you see a
meander in the Rio Grande
with a point bar on
the Mexican
side. Meanders form due to the river encountering resistance to its downslope
flow and apparently result from the river minimizing its energy. A point bar is
an area of deposition on the inner, quiet-water side of the meander. Also in the
picture is what appears to be a new channel forming. Meanders tend to grow
larger and more "loopy" with time to the point a new channel will form, cutting
across the loop and shortening the distance the water flows.

Update. I have received a clarification regarding the above
paragraph from Jeff Bennett of the National Park Service in Big Bend National
Park. I'll let him explain in his own words.

The floodplain channel on the Mexican side is not the
Rio looking for a new course, rather it is created by the outflow of a
spring. It is one of a complex of springs on both sides of the river.
Flooding in 2008 obliterated the previous channel. If you were there after
the flooding, then you might have noticed that there were two similar
springs on the river left entrance bar.

A big "thank you" to Jeff Bennett for that contribution. If any errors are
found by readers, I would very much appreciate learning about them.

A closer look at the material making up the point bars indicates it consists
of well-rounded cobbles in a matrix of sand. Obviously, the flow of the river
at the time we visited was insufficient to move this material, so the
deposition had to have occurred at times when the river was flowing much
swifter and deeper - that is, during flood stage. Were this point bar to be
buried due to some change in the current depositional/erosional pattern and
lithified (turned into rock), it would become a
conglomerate
(a rock containing abundant gravel to cobble to even larger rounded rock
pieces).

Even farther up the trail and looking upstream, you can make out old
terraces of the Rio
Grande: the mesa-like
landforms in the middle
distance. These formed when the erosional/depositional environment was
different than it is today. These terraces appear to be erosional remnants of
the previous valley surface, left as the river's downcutting power increased,
possibly due to one or more episodes of uplift (an increase in the elevation of
the surface of the land) or a change in climate. (The climate was wetter during
the last glacial
maximum, which peaked around 20 000 years ago.) From our point of view,
there appeared to
be at least two, maybe three, former valley floors now serving as tops of these
mesas. The "at least two" are likely the Qiw1 (lower) and Qiw2 (upper) terraces
of Berry and Williams
(USGS Circular
1327), dating to the middle to late
Pleistocene In the past,
they were floodplains,
such as the present floodplain: the flat area on either side of the Rio covered
with vegetation. (I need to point out that these features are in Mexico and
not included on geologic maps of the Big Bend, so I have had to exprapolate from
what is mapped on the U.S. side. However, I think the identifications made above
are correct.)

Now we are about to enter the canyon itself, and the image below is what you
see: steep walls of Santa Elena
limestone, deposited in
a sea during the Lower Cretaceous. The limestone has been exposed by the
river eating down through softer Upper Cretaceous rocks (now completely
removed at this location) and into the harder limestone. The theory is the
river channel was trapped and had no option but to continue eroding down. Its
tools were silt, sand, and gravel scraping along the river bottom. The mouth of
the canyon appears to be at the east side of a down-faulted block of rock
called a graben, which
apparently is responsible for the escarpment you encounter entering the
canyon.

If you have good powers of observation, you may have noted in the image
above what looks like a fracture in the limestone wall. If you observational
powers are sharp enough, you may even have thought it appears that the rock
has slipped along the fracture. You would be correct. What you are looking at
is one of the best examples of a
normal fault I have
ever seen. A fault is where rock slips along a fracture. A normal fault is
where the footwall
moves upward with respect to the
hanging wall (see the
figure below - the fault plane slopes away from the footwall block). There
appears to be several hundred feet of displacement along this fault. (Six
hundred feet is what I've read.) This fault appears to be a branch of the
same fault system marking the eastern edge of the graben mentioned above. The
canyon walls are about 1500 feet high.

Leaving Boquillas Canyon is as spectacular as entering it. This is the view
you get after you have gone as far as you can and turn around to leave.
Unfortunately, you have to fight the Corrizo cane (or giant cane) on this hike,
which is an exotic (not native) species that has taken over the banks. Areas of
burned cane may be due to efforts of park personnel to eradicate it. It is a
very fast growing and thirsty plant, and has been fingered as partly
responsible for the reduced flow in the Rio Grande.